The instant invention pertains to a method and apparatus for conducting geophysical exploration from a marine vessel and more particularly to such methods and apparatus in which geophysical exploration devices are deployed on cables from a boom mounted on a ship.
In marine seismic exploration, a ship tows at least one line which has one or more airguns suspended therefrom for generating a burst of seismic energy. Typically, the ship also tows a streamer cable which includes a plurality of hydrophones for detecting the earth's response to the burst of seismic energy. When an airgun is fired, seismic waves are generated which propagate through the water and into the formations beneath the water. Reflected and refracted seismic waves return from the underground formations and are detected by the hydrophones. The signals generated by the hydrophones are processed to determine the nature of the formations.
One configuration for conducting such exploration involves an array of airguns suspended on a plurality of cables which are deployed from the stern of a ship. Each cable includes several airguns which are disposed along the length thereof. In order to produce the widest possible areal array, paravanes, which include rudders for steering, are positioned at the outermost end of the outside cables in the areal array. The paravanes force the ends of the cables outwardly thus producing an areal array of airguns which is wider than when each of the airgun cables is deployed directly behind the ship from the stern thereof.
The above-described prior art technique for towing strings of airguns suffers from several disadvantages. First, if a single airgun on one of the cables should become inoperative, the entire cable must be pulled from the water to replace the inoperable airgun thereby removing several functioning guns from the array. Moreover, if the inoperable airgun is on one of the outside cables, one or more inside cables must first be removed from the water.
Another disadvantage relates to the surface area of each cable, including the airguns suspended therefrom. A long cable having several airguns suspended therefrom presents a large surface area which makes it difficult to tow the guns at lower depths. In other words, as the cable is towed, water acting against the surface area presented tends to force the cable and airguns toward the surface of the water. The faster the speed of the ship, the closer the airguns to the water surface.
Yet another disadvantage in the airgun areal arrays of the type described relates to the paravanes. In order to keep the paravanes the desired distance apart to produce an airgun areal array of a selected width, ship speed must be maintained above a minimum level. It is desirable to generate seismic energy bursts at preselected distances along the formation over which the ship moves. If the ship is traveling at too rapid a speed, reflected seismic waves from a preceding shot may not be completely recorded when it is time to generate another shot. Thus, in order to generate an accurate survey, the upper speed of the ship is limited. It may, in some instances, be desirable to utilize a wide airgun areal array in a survey which requires relatively frequent firing of the airguns. Such a survey may not be possible to conduct with the above-described prior art system due to the necessity to decrease speed in order to achieve the selected frequency of airgun firing and the necessity to increase speed in order to maintain the selected airgun areal array width.
The above-described prior art system for deploying an airgun array produces an areal array in which width of the array and depth of the airguns therein vary considerably dependent upon ship speed.
U.S. Pat. No. 4,480,574 to Bertrams discloses a ship with seismic gun jibs. In Bertrams, the ship includes a pair of booms which extend laterally therefrom at the stern. A plurality of winches are mounted on each boom at selected distances therealong and a single airgun is attached to the end of a cable wound onto each winch. A dolly which is moveable along the longitudinal axis of each boom therebeneath is provided to support an airgun being transported between the ship and one of the winches. In Bertrams, if one of the airguns becomes inoperable, the winch from which the inoperable airgun is deployed may be operated to retrieve the airgun which may be repaired or replaced without interfering with the other airguns in the array. Moreover, the Bertrams towing arrangement permits towing a areal array of airguns at a relatively slow speed without affecting the width of the airgun array. Bertrams also deploys a linear streamer from the stern of the ship which includes a plurality of hydrophones for recording the earth's response to the burst of seismic energy from the airgun array.
In addition to the cable which supports the weight of the airgun, a control line, such being generally referred to as a firing line, is connected at one end to the airgun and at the other end to equipment on board the ship. The firing line includes a high pressure hose for charging the airgun, a solenoid line for firing the gun, a depth transducer line, a gun phone line for measuring gun firing, and a stress line. In combination with a Bertrams-type boat and gun jib, it is known to extend a firing line from each air gun to a pulley mounted on the boom adjacent the winch from which the airgun associated therewith is deployed. The firing line is draped over the water between the pulley and the side of the ship from where the firing line is paid out and taken in by hand as the winch reels cable in and out.
The Bertrams arrangement does not overcome all of the problems presented by the prior art. The Bertrams air gun deployment scheme cannot create an areal array as wide as the above-described prior art technique in which paravanes are incorporated on the outer cables of the areal array. This limitation on the width of the Bertrams-type areal array is the result of the weight of the winches on the booms. A boom which is long enough to produce an areal array having a width equal to that of the prior art paravane array would not be able to support the weight of all the winches from which the individual airguns are deployed.
Bertrams provides a dolly arrangement for transporting an air gun between the ship and the boom. In order to retrieve the geophysical exploration device, i.e,, an airgun, it is necessary to pay off the cable on the winch under minimum pressure on the cable. The weight of the airgun is taken off the cable and transferred to the dolly. If the cable snags or not enough cable is payed out, the airgun can be pulled off the dolly. Moreover, a separate lifting device is necessary to move an airgun between the ship deck and the dolly.
There exists a need for a method and apparatus for deploying geophysical exploration devices from a marine vessel which overcome the above-described disadvantages inherent in the prior art. More specifically, there exists a need for such a method and apparatus in which a sufficiently wide airgun areal array may be towed at any selected speed without altering the width of the areal array and without substantially altering the depth of the individual airguns in the array. There exists a need for such a method and apparatus which incorporates improved means for handling airgun firing lines. There exists a need for such a method and apparatus in which each airgun may be safely and conveniently transported between a deployed position and the deck of the ship.